Specimens for the electron microscopy can be prepared in various ways. In order to be able to view specimens with an electron microscope, these must be processed in an appropriately defined way by uncovering the observation surface to be viewed by means of an etching method. When viewing with a scanning electron microscope (SEM), the desired surface is viewed. When viewing the specimen with a transmission electron microscope (TEM), the specimen is thinned by the etching such that electron beams can be transmitted through the specimen in the TEM, and the specimen can be viewed in transmission. Here, the quality of the image resolution essentially depends on the quality of the specimen. Therefore, the specimen should be uniformly set to an appropriately desired surface area for the SEM method or to a desired defined thickness for the TEM method by an appropriately suitable etching operation. Here, it is important that during this etching operation, the specimen structure is not varied by the operation itself. For producing such a specimen, at first a piece of the material to be examined is mechanically sawed out of the specimen body and thereafter treated by etching in order to be able to subsequently examine the specimen with an SEM or TEM. The wet chemical etching method does not lead to the desired result in this regard. For this reason, nowadays the specimens are processed under vacuum with an ion beam by ion etching, such as sputter etching, for the high-quality electron microscopic viewings with an SEM or TEM. As an ion beam, for example, an Argon ion beam having a diameter of about 1 mm is used.
Meanwhile, specimens for the electron microscopy can be prepared in different ways. Nowadays, in particular three methods are known for processing specimens by way of ion beam etching.
The ion beam slope etching technique is a method which has been used for a long time now for preparing cross-section specimens for the scanning electron microscopy (SEM). In this method, a part of the specimen surface is covered with a mask. The uncovered part of the surface is ion etched until some kind of a slope is created. At this slope, the cross-sectional structure of the specimen can be examined. The cutting depths reached lie in the range between a few 2.0 μm and a maximum of 50 μm. The preparation times vary between some minutes and several hours depending on the material and the etching depth. In order to obtain a uniformly removed cutting surface, the specimen has to oscillate during etching. This particularly applies to structured materials having highly different sputter rates, as found in the semiconductor industry. The presently known methods and techniques used for producing a slope cut only make use of one single ion source or one single ion beam.
The increasing requirements, in particular in the field of microelectronics, go in the direction of high cutting depths of up to one mm or more with, at the same time, a short preparation time and an excellent preparation quality which cannot be achieved with this known method.
A further known preparation method is the so-called wire shadow method. The wire shadow method is used for producing TEM specimens with extremely large electron transparent areas (several mm long) (Patent EP 1 505 383 A1). Here, a thin wire or a fiber is adhered onto the surface of the specimen to be etched, and the specimen is bombarded perpendicularly to the surface with a conventional ion source. By the wire shadowing, a wedge-shaped specimen is created which is electron transparent at the thinnest point. The thickness of the specimens is previously mechanically reduced to about 100 μm. In order to avoid etching selectivities in the specimen material, the specimen oscillates during the etching operation. With this method, too, the requirements go in the direction of high etching depths, with, at the same time, a short etching time and an excellent specimen quality.
These requirements cannot be met with the current etching technique both for the slope cutting method and for the wire shadow method.
A further known and commonly used preparation method is the TEM standard preparation. During the ion beam preparation of TEM standard specimens a mechanically pre-prepared specimen having a diameter of about 3 mm and a thickness of about 20 to 50 μm is etched either with two ion sources from one specimen side or one ion source on each specimen side, i.e. simultaneously on the front and the rear side. During the etching operation the specimen rotates or oscillates in order to avoid etching structures. Alternatively, the ion beam can be moved relative to the specimen or both. With this method, too, the requirements in the direction of a high etching depth with, at the same time, a short etching time and an excellent specimen quality cannot be met or only at high expense.